1. Field of the Invention
This invention relates to an injection molding composition used in producing a precision machine part having a delicate shape by injection molding an alumina powder and sintering the molded mass.
2. Description of the Prior Art
Heretofore, the method of powder metallurgy has yielded a sintered product by a procedure which comprises press molding a molding powder composition and then sintering the molded mass. It has been difficult using this method to manufacture products having three-dimensionally complicated shapes on products possessing thin-wall parts and knife-edge parts.
In order to overcome these drawbacks, a method has been proposed which obtains a sintered product by injection molding in a die of a prescribed shape an injection molding composition comprising a molding powder and a binder, heating the injection molded mass, thereby expelling the binder therefrom, and then subjecting the heated molded mass to a sintering treatment (Japanese Patent Application Disclosure SHO 57(1982)-16,103 and Japanese Patent Application Disclosure SHO 57(1982)-26,105).
Since this method requires the use of a powder having an average particle diameter of not more than 10 .mu.m, it enables manufacture of a product of high sintering density; however, it necessitates a long time for the expulsion of the binder. As a result, the cost of production is high.
If a binder having a high decomposition speed is used in order to shorten the time for the expulsion of the binder, the decomposition gas arising during the course of the binder expulsion is liable to cause flaws such as cracks, blisters and deformation in the molded mass. Although an attempt has been made to use a binder of a low decomposition speed and a binder of a high decomposition speed in a suitably mixed state, no improvement in the property of binder expulsion has been achieved.
The expression "improvement in the property of binder expulsion" as used herein refers to a decrease in the time needed to complete expulsion of the binder, a lowering of the temperature needed to effect the treatment for binder expulsion, and a reduction in flaws such as cracks, blisters, and deformation in the molded mass during the course of binder expulsion.
An object of this invention is to provide an injection molding composition which, in the injection molding of a precision part from alumina powder as a raw material, contributes advantageously to improving the property of binder expulsion which the conventional composition has failed to attain.